Description of the Prior Art
Recently, Ito et al. have disclosed in JP A 1-14276 a thermoreversible acrylamide polymer thickener, for example, a temperature-sensitive gelling composition comprising poly(N-isopropyl acrylamide) and a resin emulsion. Kimura et al. disclose in JP A 4-261453 a temperature-sensitive gelling composition comprising a resin latex having a chemical stability index of not more than 24, a polyvalent metal salt and an adduct of an alkylene oxide of an alkylphenol-formalin condensate, which adduct is a non-thermoreversible thickener. However, while disclosing, as a resin latex having a specific chemical stability index, a latex obtained by polymerizing vinyl monomers by using a smaller amount of a surface active agent or a self-emulsified latex obtained by using a polymerizable emulsifying agent, they make no mention of the amount of the emulsifier in the aqueous phase of the latex. The term "chemical stability index" means an index showing the chemical stability of a latex as expressed by the time required for the latex to coagulate and solidify after an aqueous solution of aluminum sulfate has been added thereinto. The actual method of measuring the index will be explained in the Reference Example 1.
By the way, in order that polymer particles be stably dispersed in an aqueous phase of a latex, emulsifier molecules need only to be chemically or physically bound to the polymer particles and need not exist in the aqueous phase. In order to impart a temperature-sensitive property to a latex and further strengthen the property, it is rather desirable that emulsifiers do not exist in the aqueous phase. In many applications of the binder compositions where a temperature-sensitive gelling property is required, a various kind of auxiliary agents which are water-soluble or water-dispersible and also ionic are frequently compounded therewith, thus the stability of the latex to factors other than heat tends to be impaired. This leads to a tendency to use a polymerizable or non-polymerizable emulsifier, in preparing the latex, in an amount larger than required, leaving an excessive amount of the emulsifier remaining in the aqueous phase. Accordingly, in the prior arts mentioned above, chemical stability to the factors other than heat and improvements in gloss, printability and color development property in the practical application remain insufficient. Thus, the conventional temperature-sensitive gelling compositions are not good enough in exhibiting a temperature-sensitive gelling property, and, therefore, the use thereof in coating paper gave problems that 1 no excellent gloss was obtained, because pigments such as clay and calcium carbonate in the color coating composition could not be satisfactorily orientated by calendaring and 2 no satisfactory printability was obtained, because the resin concentration on the surface of the coated layer became much higher than it was required. There was also a problem that no satisfactory color development was obtained when the compositions were applied to pressure-sensitive recording sheet or heat-sensitive recording sheet uses.